A comparison of two coastal barrier jet events along the southeast Alaskan coast during the SARJET Field Experiment

Olson, J.B., B.A. Colle, N.A. Bond, and N. Winstead

Mon. Wea. Rev., 135(8), 2973–2994, doi: 10.1175/MWR3448.1 (2007)

The Southeastern Alaskan Regional Jets experiment investigated the structures and physical processes of barrier jets along the coastal Fairweather Mountains near Juneau, Alaska, from 24 September to 21 October 2004. This paper compares in situ aircraft data and high-resolution simulations from the first intensive observation period (IOP1), which featured a nearly terrain-parallel barrier jet (classical jet) with another coastal jet event (IOP7) that was influenced by offshore-directed gap flows at the coast (hybrid jet). IOP1 featured southerly onshore flow preceding a landfalling trough, which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5–10 m s−1 wind enhancement in the alongshore direction in the lowest 1 km MSL. In contrast, IOP7 featured higher surface pressure and colder low-level temperatures to the east (inland) of the study area than did IOP1, which resulted in offshore-directed coastal gap flow exiting Cross Sound below ∼500 m that turned anticyclonically and merged with the ambient flow. Unlike the classical jet (IOP1), IOP7 had a surface warm anomaly adjacent to the steep coastal terrain, while a cold anomaly existed farther offshore within the gap outflow. Above the shallow gap flow (>500 m MSL), there were more classical barrier jet characteristics. High-resolution fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) simulations were performed to compare the structures and underlying dynamics between the two cases. Model trajectories show that coastal winds for IOP1 originated offshore, while much of the coastal flow in IOP7 had gap flow origins near the surface and offshore origins above the gap outflow. A model momentum budget suggests that the vertical mixing of southerly momentum from aloft forced the gap outflow in IOP7 to turn anticyclonically more rapidly than an inertial circle. A simulation of IOP7 with the Cross Sound gap removed (filled in) produced a coastal jet with similar maximum wind speeds to the control but resulted in a reduction in the width of the coastal jet by about 40%.